Liquefied natural gas (LNG) is natural gas, primarily methane, that has been converted to liquid form for ease of storage or transport. The liquefaction gas pretreatment process involves removal of certain components such as acid gases, mercury, water and heavier hydrocarbons. The natural gas is condensed into a liquid at close to atmospheric pressure by cooling it to about −162° C. Specially designed ships are used for the shipment of LNG over routes where pipelines do not exist to transport the natural gas in liquid form. Due to the abundance of natural gas in North America there is a shift occurring from importing LNG to exploring the option of exporting LNG. A network of natural gas pipelines exist to supply feed gas to an LNG plant. It has been found that the pipeline feed gas may contain oxygen and various sulfur contaminants which can cause operating issues for adsorbent based systems such as a molecular sieve dehydration unit.
In an adsorbent based system if oxygen is present with sulfur compounds in the regeneration gas the oxygen and sulfur can react during the heating and cooling steps forming elemental sulfur and sulfates. Sulfates block the molecular sieve pores and permanently deactivate the adsorbent resulting in short bed life. Additionally, elemental sulfur may be formed that can plug passages of the regeneration gas cooler or other equipment and lead to fouling and poor performance.
Natural gas is dehydrated in adsorbent beds for the purpose of protecting the downstream cryogenic LNG plant from hydrate formation. In a conventional prior art design, water is removed in a first adsorbent bed resulting in a dry product stream. Then a slip stream of product gas is first heated and sent to regenerate an adsorbent bed and produce a gas which contains desorbed impurities including water. The water may be removed from this gas stream by being sent through a cooler to condense water then to a knock out drum for vapor liquid separation. The gas can be recycled back to be combined with the wet feed that is sent through the first adsorbent bed.
It has been found that operation of thermally regenerated adsorbent based dehydration units in the presence of oxygen and sulfur compounds resulted in formation of elemental sulfur and sulfur compounds during the regeneration step. The sulfur rapidly accumulated on the molecular sieve bed, and caused an accelerated loss of dehydration capacity, necessitating replacement of the molecular sieve within a matter of months. This failure involving the formation of sulfur and sulfates was found to occur despite attempts to modify conventional designs through the use of an adsorbent having a lower regeneration temperature based upon the assumption that such lower regeneration temperatures would be adequate to prevent the reaction of oxygen and sulfur to produce elemental sulfur and sulfates. In the present invention, a process design has been developed that prevents the formation of undesired sulfur and sulfates from the reaction of oxygen and sulfur.